This invention relates to measuring contact potential differences.
A contact potential difference (CPD) is a difference between an electrostatic potential at a surface of a sample and a contact potential of a metal electrode of a CPD probe, which is determined by the probe electrode""s work function. CPD measurements are non-contact measurements that are particularly useful for characterizing numerous structures extensively used in semiconductor electronics, such as dielectric layers disposed on semiconductor substrates. Examples of these applications are described by J. Lagowski and P. Edelman in xe2x80x9cContact Potential Difference Methods for Full Wafer Characterization of Oxidized Silicon,xe2x80x9d Inst. Phys. Conf., Ser. No. 160, p. 133-144 (1997), and by D. K. Schroder in xe2x80x9cContactless Surface Charge Semiconductor Characterization,xe2x80x9d Material Science and Engineering, B91-92, p. 196-210 (2002). In cases where the sample is a dielectric film on a semiconductor substrate the contact potential difference of the sample, VCPDS, can be expressed as:
VCPDS=VSxe2x88x92xcfx86el,
where xcfx86el is the contact potential of the CPD probe electrode and VS is the sample surface potential:
VS=Vdiel+VSB+xcfx86s.
Here, Vdiel is the potential drop across the dielectric layer, VSB is the semiconductor surface barrier, and xcfx86s is the contact potential corresponding to the semiconductor work function at the flatband condition (i.e., when VSB=0).
Electrical charge residing in a dielectric layer, on the surface of the dielectric layer, or at the interface between the dielectric layer and the semiconductor substrate can be monitored by measuring a change in a VCPDS in response to an electric charge, xcex94Q, intentionally placed on the dielectric layer""s surface, for example, by a corona discharge in air. This change in VCPDS, can be expressed as:
xe2x80x83xcex94VCPDS=xcex94Vdiel+xcex94VSB,
where xcex94Vdiel=xcex94Q/Cdiel, Cdiel being the dielectric layer capacitance. xcex94VSB=xcex94Q/(CSC+Cit),
where CSC and Cit are the capacitance of the semiconductor space charge and interface traps, respectively.
Electrical current in the dielectric layer can also be monitored by measuring a rate of change of VCPDS, after corona charging of dielectric, dVCPDS/dt. In this type of measurement, VCPDS is recorded as a function of time. A current, J, is obtained from the rate of change of the voltage across the dielectric layer, Vdiel:   J  =                    C        diel            ⁢                        ⅆ                      V            diel                                    ⅆ          t                      ≈                  C        diel            ⁢                                    ⅆ                          V              CPDS                                            ⅆ            t                          .            
Key properties of dielectrics (e.g., electrical conductance, charge trapping) important for semiconductor device functioning are temperature dependent. Therefore, characterization of dielectrics would clearly benefit if CPD could be measured over wide temperature range including elevated temperature as high as 400xc2x0 C. or even 500xc2x0 C.
Typical CPD probes incorporate elements such as a measuring electrode, an operational FET preamplifier, an electromagnetic or piezoelectric vibrator, soldered electric wires, elements connected with glue or epoxy. These elements can be affected, or even destroyed, by elevated temperature. For example, currently manufactured probes would generally fail at temperatures in excess of 400xc2x0 C.
Measurement systems can be designed to avoid overheating of the probe during measurement of samples at elevated temperature, without any modification of the probe assembly and without a need for probe cooling devices that would stabilize the probe temperature during a measurement of hot samples. To avoid overheating, the probe is cycled between two positions: a room temperature position in the proximity of a reference plate; and a xe2x80x9chotxe2x80x9d position in the proximity of sample at elevated temperature. The cycle can be asymmetric in time. For example, for the majority of a typical cycle lasting about 15 seconds, the probe is in a room temperature position. The time the probe spends in this position (e.g., about 10 seconds) is referred to as resting time xcex94trest. For a short portion of the cycle (e.g., about 2 seconds), referred to as measuring time, xcex94tmeasure, the probe is in the proximity of the sample at elevated temperature. This cycle limits the heating of the probe while it measures the sample and helps to cool the probe back to a room temperature while the probe is in the proximity of a reference plate.
When the sample is at high temperature, such as 400xc2x0 C. or 500xc2x0 C., a noticeable heating of the probe can take place even during a short 2-second sample measuring time. This can alter the contact potential, xcfx86el, of the probe electrode, and change the probe reading of the sample VCPDS=VSxe2x88x92xcfx86el. The present method makes the sample measurements substantially independent of changes in xcfx86el. In other words, the described method provides compensation for any changes in xcfx86el that may occur due to probe heating. This is done using two measurements: the contact potential of the sample VCPDS=VSxe2x88x92xcfx86el, which is measured with the probe positions in the proximity of the sample; and, the measurement of a reference plate that is done immediately after returning of the probe to position in the proximity of a reference plate. The second measurement provides VCPDR=xcfx86refxe2x88x92xcfx86el, where xcfx86ref is the contact potential of the reference plate. From these two measurements a difference is obtained xcex94VCPD=VCPDSxe2x88x92VCPDR=VSxe2x88x92xcfx86ref, xcfx86ref is constant because the reference plate is kept at a constant reference temperature (e.g., typically room temperature). Thus, xcex94VCPD provides an accurate measure of the sample contact potential, VS, that is not substantially affected by any changes in xcfx86el.
While the described methods and systems focus on elevated temperature measurement done with CPD probes, it shall be pointed out that the methods can be applied to measurement with any non-contact probe that may be affected by exposure to elevated temperature. Such probes may include, for example, photovoltaic probes for the surface photovoltage measurement, or optical probes for probing light reflected from the sample or emitted by the sample.
In general, in a first aspect, the invention features a method for elevated sample temperature measurement. The method includes heating a sample to a sample temperature, T, and moving a probe from a first position to a second position, wherein the first position is proximate to a reference plate held at constant temperature, T0, and the second position is proximate to the sample, and T is greater than T0. The method further includes measuring a contact potential difference of the sample, VCPDS, with the probe being held in the second position for a measuring time, xcex94tmeasure, sufficiently short to prevent substantial heating of the probe. The method also includes returning the probe to the first position, measuring a contact potential difference of the reference plate, VCPDR, and determining a difference xcex94VCPD=VCPDSxe2x88x92VCPDR as a measure of a sample contact potential at T.
Implementations of the method can include one or more of the following features.
The probe need not be actively cooled while in the second position. xcex94tmeasure can be 2 seconds or less.
The method can further include holding the probe in the first position for a probe resting period, xcex94trest, of 5 seconds or more after returning the probe to the first position.
T0 can be less than 100xc2x0 C. (e.g., less than 80xc2x0 C., less than 50xc2x0 C., less than 30xc2x0 C., less than 25xc2x0 C., such as 23xc2x0 C.). During the measurement of VCPDS and VCPDR, the temperature of the probe can be kept within 5xc2x0 C. of T0 (e.g., within 3xc2x0 C. of T0, within 2xc2x0 C. of T0, within 1xc2x0 C. of T0). The sample temperature T can be between T0 and 500xc2x0 C.
The sample can include a dielectric layer. The reference plate can include gold or platinum.
The method can also include cycling the probe between the first position and the second position, and during each cycle, measuring VCPDS in the second position and VCPDR in the first position, and determining a sample contact potential difference from a difference between VCPDS and VCPDR from each cycle. In some embodiments, the method can also include changing the sample temperature between measuring VCPDS of successive cycles and measuring the sample temperature, T, each time the probe measures VCPDS. Alternatively, or additionally, the method can further include determining a dependence of A VCPD on the sample temperature. A corona charge can be deposited on sample surface prior to changing the sample temperature.
Once the system acquires data from cycling the probe between the first and second positions, the method can include characterizing the sample in one or more ways. For example, the method can include identifying contaminant ions present in the sample from the dependence of xcex94VCPD on the sample temperature. Alternatively, or additionally, the method can include determining the concentration of each contaminant ion in the sample from the dependence of xcex94VCPD on the sample temperature. As another example, the method can include monitoring desorption of contaminants from the sample from a rate of change of the dependence of xcex94VCPD on the sample temperature.
In a second aspect, the invention features a system, including a sample stage for supporting a sample, a heating element for heating the sample to a sample temperature, a reference, a probe for making contact potential difference measurements mounted on a probe arm, and an electronic controller, which during operation causes the probe arm to position the probe relative the sample to measure a contact potential difference between the probe and the sample, and then causes the probe arm to position the probe relative to the reference and to measure a second contact potential difference between the probe and the reference.
The system can be adapted to implement the methods of other aspects of the invention. The system can also include one or more of the following features.
The system can include a cooling element positioned relative the reference sample, and during operation, the cooling element can stabilize the reference plate temperature. The probe can be a Kelvin probe, a Monroe probe, or a Trek probe.
In a third aspect, the invention features a method, including moving a probe at a first temperature from a first position to a second position, wherein the first position is proximate to a reference at the first temperature and the second position is proximate to a sample, the sample being heated to a sample temperature greater than the first temperate, measuring a first contact potential difference of the sample with the probe in the second position, returning the probe to the first position, and measuring a second contact potential difference of the reference, wherein the probe is held in the second position for a period wherein the probe""s temperature is substantially unchanged from the first temperature during the measuring.
Implementations of the method can include any of the features of the other aspects of the invention.
In a fourth aspect, the invention features a method, including moving a probe at a first temperature from a first position to a second position, wherein the first position is proximate to a reference at the first temperature and the second position is proximate to a sample, the sample being heated to a sample temperature greater than the first temperate, measuring a first contact potential difference of the sample with the probe in the second position, and removing the probe from the second position. The probe is held in the second position for a sufficiently short time so that the probe""s temperature is substantially unchanged from the first temperature during the measuring.
Implementations of the method can include any of the features described in regard to other aspects of the invention. Implementations of the method can also include one or more of the following features.
The method can include measuring a second contact potential difference of the reference with the probe in the first position. Additionally, the method can include determining the sample contact potential difference from the first and second contact potential differences.
After positioning the probe in the first position, the first contact potential difference can be measured before the sample can substantially heat the probe.
The method can also include returning the probe to the first position after removing the probe from the second position.
Note that substantial heating of the probe includes any heating that would damage the probe and/or cause errors in the measurement that could not easily be corrected for to provide measurements within the desired accuracy of the particular application (e.g., errors that are not corrected for by the methods disclosed herein). Substantial heating can include, for example, heating that would change the probe temperature more than 10xc2x0 C. from T0.
Embodiments of the invention can include one or more of the following advantages.
Commercial CPD probes designed for operation at or near room temperature (e.g., between 20xc2x0 C. and 25xc2x0 C.) can be used without any modification while measuring samples at elevated temperatures (e.g., more than 100xc2x0 C., such as more than 200xc2x0 C., 300xc2x0 C., 400xc2x0 C., or even 500xc2x0 C.). Effects of probe heating on the CPD measurements can be corrected for using a reference measurement, providing precise CPD measurements of samples at elevated temperatures.
The measurement techniques enable CPD characterization of samples as a function of sample temperature. For example, these techniques can be used to monitor electrical current in a dielectric as a function of temperature. Transport of ionic contaminants in a dielectric can also be monitored. The temperature dependence of contaminant transport can be used to determine the type and concentration of the contaminant.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and apparatus similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and apparatus are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the apparatus, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.